Aggression is an innate behavioral trait that has numerous ecological functions. Basic research in understanding the neurobiology of aggression has been performed using both invertebrate and vertebrate model systems. Despite extensive study, relatively little is known regarding how specific molecular/cellular factors organize the developing nervous system to program aggressive behavior. The fruit fly (Drosophila melanogaster) genome is fully sequenced, and as a model system, fruit flies offer numerous possibilities for behavioral and genetic manipulation, including sophisticated imaging techniques. This proposal will use Drosophila as a model system to study the neurochemical genetic organization of the brain and its relevance to programming social behavior. The display of aggression and courtship behavior in the fruit fly is sexually dimorphic - with males courting females and using different patterns of fighting behavior than females. These behavioral sex differences are under the direct control of genes of the sex determination hierarchy, including fruitless (fru). The fruitless gene displays sex- specific alternative splicing. Females make a truncated message that makes no protein. If females make the male-spliced variant, they display male-like courtship and aggressive behaviors. Males that do not make the male-spliced variants do not court females, display female patterns of fighting, and are infertile. Until recently, the specific neurohormonal/neurotransmitter systems that show co-expression of fru were unknown. Interestingly, the phenol analogue of norepinephrine - octopamine (OA), which is commonly found in invertebrate species, was recently found to play a critical role in decisions made by male flies to either court or fight. Approximately 100 OA neurons are found in the fly brain. Only three of these OA neurons co-express the male forms of fru (FruM). The results from previous work have demonstrated that these FruM/OA cells are involved in decision making by males between courtship and aggression. However, the circuitry involved with these FruM/OA neurons and their specific physiological roles in coordinating sensory input and motor output to drive social behavior in flies is unclear. By utilizing a combinatorial genetic approach, the results from this proposal should: first, allow the construction of a database of crosses between lines that will allow for direct manipulation of genes in individual or small groups of OA neurons; and then, provide a model to examine the behavioral consequences of knocking out, enhancing or reducing the function of these neurons. This research program represents first steps in elaborating the neural circuitry underlying social behavior in fruit flies.